1. Field of the Invention
The present invention relates to a detection and determination of data within a range having a low signal-to-noise ratio (referred to as a S/N ratio hereinafter) in a communication or a recording device etc., and more particularly, relates to a carrier phase synchronous type maximum likelihood decoder for transmitting and receiving digital signals.
2. Description of the Related Art
In recent years, it has been noticed the receiving system in which modulation signals, in a broad sense, involving the phase modulation of convolutionally encoded data, an orthogonal amplitude modulation, an encoded modulation, CPFSK and a modulation having an interference between the symbols, are detected with Maximum Likelihood Sequence Estimation Scheme, because it provides the satisfactory S/N ratio against error rate characteristics.
The maximum likelihood sequence estimation system as mentioned above, is structured to estimate a transmit data sequence with a minimum error on the basis of reference signals which are hypothetical ideally detected signals.
Since the maximum likelihood estimation system has the advantage that the remarkable low error rate can be expected in the low S/N ratio range which the other system cannot achieve, it has been considered widely in various fields.
The demodulation means is required to have a low error rate in such low S/N ratio. To carry out a maximum likelihood decode, coherent detection means which has good error rate characteristics is utilized.
FIG. 13 is a block diagram showing the structure of the phase synchronous circuit used in such a synchronous detection system. As shown in the drawing, the phase synchronous circuit includes an orthogonal demodulate circuit 1, a temporary detect circuit 3, a phase error detect circuit 5, a regenerated carrier wave generate circuit 7 and a maximum likelihood sequence estimate circuit 9.
The orthogonal demodulate circuit 1 demodulates orthogonally the received signals. The temporary detect circuit 3 detects the transmitted data with immediate detection scheme. The phase error detect circuit 5 detects the phase error between the output of the orthogonal demodulation circuit 1 and the output of the temporary determine circuit 3.
The regenerated carrier wave generate circuit 7 generates the regenerated carrier wave controlled by the output of the phase error detect circuit 5. The maximum likelihood sequence estimate circuit 9 estimates the transmit signal with maximum likelihood on the basis of the output signal of the orthogonal demodulation circuit 1.
In the phase synchronous circuit as mentioned above, the phase error between the regenerated carrier wave and the carrier wave of receive signal is calculated and the phase of the regenerated carrier wave is controlled based on the phase error. This loop does not contain a long delay, and shows good tracking performance in high SN ratio condition.
However, in such a phase synchronous loop, when "the output of temporary detect circuit" is not correct, the incorrect phase error is calculated and the phase of regenerated carrier wave is controlled on the basis of the incorrect phase error, therefore, the synchronization cannot be achieved well, especially in low SN ratio condition. The phase error calculated on the basis of the correct transmit data is the error where a Gaussian random noise is added to the real phase error. Therefore, the noise component can be removed by means of a linear filter, if the phase movement of the carrier wave is within a certain range. On the contrary, since the phase error calculated on the basis of the incorrect transmitted data has not only the Gaussian random noise added to the real phase error, but also the incorrect error based on the erroneous detection, which acts as a nonlinear noise, it is difficult to hold the synchronization when the error exceeds a certain range. Further, it is very difficult to synchronize when the phase of transmit carrier wave changes fastly, for example by fading.
Since the phase error is calculated on the basis of the temporary detected data involving larger amount of errors than the maximum likelihood sequence, the sufficient operation cannot be carried out in a low S/N ratio. As a result, it is not possible to obtain good error rate performance in a low S/N ratio.
FIG. 14 is a block diagram showing the structure of another phase synchronous circuit. This phase synchronous circuit includes an orthogonal demodulate circuit 1, a maximum likelihood sequence estimate circuit 11, a delay circuit 13, a phase error detect circuit 5 and a regenerated carrier wave generate circuit 7.
The delay circuit 13 delays an output signal of the orthogonal demodulate circuit 1. The maximum likelihood sequence estimate circuit 11 estimates the maximum likelihood sequence of the transmit signal on the basis of the output signal from the orthogonal circuit 1. The phase error detect circuit 5 detects the phase errors between the outputs of the delay circuit 13 and the Maximum likelihood sequence estimate circuit 11. The regenerated carrier wave generate circuit 7 generates the regenerated carrier wave on the basis of the output from the phase error detect circuit 5.
In the phase synchronous circuit as mentioned above, the phase error is calculated by the phase error detect circuit 5 on the basis of the output of the maximum likelihood sequence estimate circuit 11. The most reliable phase errors can be calculated by the system described above.
However, for determining the maximum likelihood of the transmit data, it takes a quite long time.
Further, the time for decoding is taken longer in some case. For example, in decoding such punctured convolution data of the 7/8 encode rate, it takes in general the time nearly ten times as much as the constrained length. The circuit for adjusting the regenerated carrier wave by utilizing the phase error calculated on the basis of the transmit data determined in a long period of time has the remarkably large delay time in the control loop, therefore, it is impossible to attain a good characteristics in the phase tracking. Namely, the circuit has disadvantages that a frequency tracking range becomes remarkably narrow, a loop becomes easily unstable and a time required for the synchronization is taken remarkably long.
In the phase synchronization circuit, the delay in the loop becomes longer, the sufficient phase tracking characteristics cannot be obtained. Therefore, the synchronization failure in the loop is caused if phase of the carrier wave varies even slightly. As a result, it is not possible to obtain good error rate performance.
As described above, it is hard to carry out the maximum likelihood sequence estimation and achieve the phase synchronization of signals having the large Inter-Symbol-Interference simultaneously. Moreover, the phase synchronization in the low S/N ratio is remarkably hard to obtain. Further, when the maximum likelihood sequence estimation system is used in the satellite mobile communication and the land mobile communication, it is hard to obtain the carrier phase synchronization, because the carrier phase is remarkably fluctuated by the complicated transmission path characteristics, such as a multipath fading and a shadowing. Further, the ratio of whole signals occupied by the preamble or the pilot is remarkably high, therefore, it is difficult to utilize the frequency band efficiently.